This is an investigation of the role of (junctional) cell-to-cell communication in cancer growth and the role of communication defects in the etiology of cancerous growth. Sensitive fluorescent tracer techniques were developed for detecting deviations of normal communication. Such deviations were found in various Rous sarcoma-transformed cell types. A control of communication by hormones (catecholamines and prostaglandins) was found. (A)